The present invention relates generally to proximity detection and level measurement and, more particularly, to electronic circuits used in capacitive sensors that determine the presence or level of an object, fluid or materials.
A device or system having the ability to detect and/or measure the presence, level, or quantity of particular materials, commonly referred to as a proximity detector or level sensor, has many uses. For example, proximity detectors may be used to detect or sense the level of grain, aggregate, fluids or other materials in a storage container, or to detect the presence of a metal part on a production line. Capacitive sensors are extensively used for proximity detection and level measurement. A conventional capacitive sensor, which includes one or more conductive plates, is sensitive to changes in the dielectric constants of materials or fluids, and detects the presence, or lack thereof, of material in the vicinity of the plates by measuring the capacitance between the plates, which is proportional to the dielectric constant of the material filling the space between the plates. Similarly, another conventional form of capacitive sensor, which uses a sensing antenna, e.g., a long wire or strip immersed into a tank or storage bin holding a variable level of fluid or material, measures the level of the fluid or material by sensing and measuring the capacitance of the sensing antenna.
Designing and producing proximity sensors and level sensors for use in detecting and/or measuring high dielectric substances, e.g., water, salt water, and certain plastics, is relatively straightforward because the change in capacitance of even modest-sized sensing plates is large. There is a continuing need, however, for a proximity detector or level sensor capable of detecting and/or measuring materials having a low dielectric constant, such as, for example, grain, feed, diesel fuel, and gasoline. Such low dielectric constant materials are difficult to detect or measure using conventional capacitive sensors because the relatively small changes in capacitance due to the material or fluid can become hidden by drift of the sensor electronics due to, for example, variations in power supply voltages. The detection and measurement of these low dielectric materials and fluids requires exceptionally stable circuitry to make such measurements practical. Thus, there is a continuing need for a capacitive sensor that is less susceptible to certain environmental conditions such as temperature variations and variations in the components used in the electronic circuitry associated with the sensor.
Calvin U.S. Pat. No. 4,345,167 illustrates one prior art electronic circuit used in a switched capacitive sensor. Such a circuit is illustrated in prior art FIG. 1, in which the electronic switch, oscillates between the reference voltage node V1 and the node connected to the current-to-voltage circuitry at a particular frequency. Thus, the sensing antenna is repetitively charged to the reference voltage V1 and then discharged, generating a current proportional to the amount of charge on the antenna. The current-to-voltage circuitry generates an output voltage Vo, that is proportional to the capacitance on the sensing antenna. A variation is illustrated in Philipp U.S. Pat. No. 5,730,165, which discloses a charge integrating capacitor, a simplified version of which is illustrated in prior art FIG. 2. In this circuit, the switch is cycled a predetermined number of times, injecting charge into a capacitor, which increases the voltage across the capacitor slightly each cycle. The amplifier measures the voltage across the capacitor after the switch has cycled the predetermined number of times. Each of these two prior art circuits operate by alternatively charging and discharging a sensing plate or antenna between two voltages. The electronic charge required to effect this change in voltage is proportional to the capacitance of the plate or antenna and the magnitude of the voltage change according to the equation, q=xcex94VC. The capacitance between the two electrodes (e.g., plates) is proportional to the dielectric constant of the materials and/or fluids in the vicinity of the electrodes. The capacitance is based on the geometry of the electrodes and may be represented by, C=∈rC0, where C0 is the capacitance between the electrodes with only air present therebetween, and ∈r is the effective relative dielectric constant of the material and/or fluid between the electrodes. If the material or fluid does not completely fill the space between the electrodes, then the effective dielectric constant will be reduced from that of the material or fluid. The effective dielectric constant for the electrode arrangement, however, does not need to be calculated, only the change in capacitance of the sensor.
The prior art switched capacitor or charge pump sensor circuits measure the change in capacitance by measuring the change in charge required to change the voltage on the sensing electrode, by alternatively connecting the sensing electrode to a reference voltage, V1, and then to a second voltage, V2 (which may be ground). This change in charge may be represented by:
xcex94q=(V1xe2x88x92V2)∈rC0
After the electrode has charged to V2, it is immediately re-connected to the reference voltage, V1. If this switching sequence is repeated at some frequency ƒ, then the repetitive injection of this charge xcex94q represents a current that is proportional to the effective dielectric constant according to:
I(∈r)=(V1xe2x88x92V2) ∈rC0ƒ
This prior art sensing technique can be used as a proximity detector by merely detecting when the current exceeds a certain threshold value. Alternatively, if the conventional two plate sensing electrode is replaced by a wire (or pair of wires) immersed into a tank of fluid or granular material, the capacitance of the wire will vary as the level of material changes. This latter arrangement results in a material or fluid level sensor.
When using proximity detectors or level sensors with materials or fluids having high dielectric constants, such as aqueous solutions or pellets of high dielectric constant plastics, the change in capacitance, and therefore the change in current, is very pronounced and relatively easy to detect. For low dielectric constant materials or fluids, such as, for example, grain, feed, diesel fuel, gasoline, and certain low dielectric plastics, the change in capacitance is very slight, for example only a change of a few percent (as compared to doubling or tripling the capacitance in the high dielectric constant applications). This results in a relatively large current without the material present and only a slightly larger current when the material to be detected is present. To make this small change in current more apparent in some devices, a fixed current (which may be approximated by a resistor coupled between the second reference voltage and ground potential) is typically subtracted from the signal current. Thus, if the fixed current is adjusted to be approximately equal to the current when no material is present, then the material will produce a large relative change in net current.
Each of these prior art techniques is dependent on the stability of the reference supply voltage. The sensor current depends directly on the magnitude of the change in voltage. As the reference supply voltage fluctuates, the sensor current will also fluctuate. This dependence on the stability of the reference supply voltages is a limitation of this technique. A need remains, therefore, for a circuit used in capacitive sensors that is less susceptible to fluctuations in the reference supply voltages. Preferably, the circuit will directly and automatically compensate for variations in the reference supply voltages.
An improved circuit for detecting the change in capacitance of a sensing electrode that is less sensitive to fluctuations in supply voltage has now been developed. The circuit substantially improves the accuracy of a proximity detector or level sensor while minimizing adverse effects due to ambient conditions and electrical component variations. The present invention directly and automatically compensates for expected variations in the reference voltages commonly used in conventional switched capacitor or charge pump capacitive sensors. The circuit described herein controls a current source so that it varies proportionally to the supply voltages governing the input voltages to a comparator or differential amplifier. This results in a sensor that is an order of magnitude more stable over temperature, reference supply voltage fluctuations, and certain component parameter drifts over the usable life of the sensor. As a result of this improved stability, small capacitances can be reliably discerned, or conversely, low dielectric constant materials and fluids can be sensed and/or measured. Using the novel circuit described herein, proximity sensors and material or fluid level sensors can be designed that can sense or measure materials or fluids that previously had been too difficult to detect and/or measure reliably.
In one embodiment of a balanced charge pump circuit disclosed herein for measuring the capacitance of a sensor element, the capacitive sensor element is coupled through a switching circuit. The switching circuit preferably has a charging state during which the sensing element is connected to a relatively fixed upper reference voltage source. The switching circuit also preferably has a discharging state during which the sensing element is connected to a summing node, typically at some different voltage between the upper and lower reference voltages. The switching circuit also preferably assures that the sensing element is disconnected from both the upper reference voltage source and the summing node at least briefly between these two connected states. As a result of this switching action, a packet of charge is transferred from the higher voltage connection to the lower voltage connection, the quantity of charge depending on the voltage difference between these two connections and the capacitance of the sensing element at the time the connections are changed.
The balanced charge pump circuit includes a summing node coupled to the sensor element during one of the switching states described above. The pump circuit also includes a current source, such as a current drain resistor, having one terminal connected to the summing node and the other terminal connected to a fixed lower reference voltage source. This lower reference voltage source is maintained at approximately the same voltage with respect to the upper reference voltage source using conventional voltage regulators, batteries, and the like. This drain resistor is designed to drain away from the summing node, on average, exactly the same quantity of charge per second as the sensing element and switching circuits are transferring into the summing node at some particular exact value of sensing element capacitance when the summing node is at some particular design voltage. The circuit also includes a differential amplifier having a non-inverting input terminal and an inverting input terminal, in which inverting input terminal is coupled to the summing node. A feedback impedance, typically either a resistance or a capacitance, or both, may be included to connect the output of the differential amplifier to the inverting input. This feedback impedance contributes an additional current to the summing node and therefore affects the voltage of the summing node along with the sensing element and switches and the drain resistor. In addition, a voltage divider, which may be a pair of voltage dividing resistors arranged in series, is coupled to the non-inverting input terminal of the differential amplifier to maintain the voltage at the non-inverting input terminal equivalent to a proportion of the voltage of the upper reference voltage source. The differential amplifier generates a voltage differential signal at an output terminal representative of the discharge current from, and the capacitance of, the sensor element. A charge integrating capacitor is preferably coupled between the summing node and the non-inverting input terminal of the differential amplifier to smooth out the current pulses flowing into the summing node each time the sensing element is connected to the summing node.
The balanced charge pump circuit may be used as part of a proximity detection or level measurement system in which the sensor element is positioned within a storage tank to detect or measure the level of fluid or materials stored within the storage tank. The sensor element may include spaced apart first and second electrodes and be positioned within the storage tank to receive the fluid or materials between the electrodes during use. The capacitance between the electrodes will then vary as a function of the fluid or material level height in the storage tank. The balanced charge pump circuit produces an output signal indicative of the level of fluid or materials contained within a storage tank.